El Niño Southern Oscillation

The El Niño Southern Oscillation (ENSO) is an irregular, recurring climate change phenomenon that is associated with variations in sea surface temperatures in the equatorial Pacific Ocean. It is caused by changes in the strength and direction of the Walker circulation, which governs zonal and vertical atmospheric circulation between the tropical eastern and western Pacific Ocean. The ENSO is characterized by a warming phase called El Niño (‘the boy’ in Spanish) and a cooling phase called La Niña (‘the girl’ in Spanish). By affecting the distribution of heat and precipitation across the Pacific Ocean, the ENSO is able to greatly influence weather and climate in many regions around the world.

The Walker circulation generally arises from a high air pressure system above the eastern Pacific Ocean near South America and a low air pressure system above the western Pacific Ocean near Indonesia and Australia. Under normal conditions, easterly equatorial winds result in the development of a warm water pool towards the western Pacific.

An El Niño may occur when the Walker circulation weakens or reverses, resulting in a lower air pressure above the eastern Pacific and a higher air pressure above the western Pacific. During El Niño, the Pacific warm water pool moves east towards South America and the surface waters of the eastern Pacific warm up as the upwelling of colder deep waters is reduced. Therefore, an El Niño is characterized by warmer and wetter climates in South America and colder and drier climates in Indonesia and Australia.

A La Niña may occur when the Walker circulation grows especially strong. During La Niña, the Pacific warm water pool moves further west towards Indonesia and Australia and the surface waters of the eastern Pacific cool down as the upwelling of colder deep waters is enhanced. As a consequence, a La Niña is characterized by colder and drier climates in South America and warmer and wetter climates in Indonesia and Australia.

El Niño and La Niña may vary in duration and intensity, but each phase generally lasts one to a few years. Extreme shifts of the ENSO may result in severe floods or droughts and may therefore have a large impact on society. Because the occurrence of El Niño and La Niña is irregular, the ENSO is difficult to predict longer than a year in advance.